This invention relates to an automated system for dispensing and diluting a sample, which is capable of dispensing an accurate volume of the sample. More particularly, this invention relates to a system which is capable of collecting an accurate volume of the sample by an accurate detection of the sample liquid surface, and which is capable of introducing all of the thus collected accurate volume of the sample together with a diluting solution into the reaction vessel to thereby consistently enable an accurate volume of the sample to be dispensed.
In the fields of clinical examination, biochemistry, biotechnology, chemical analysis and the like wherein tests involving chemical reactions are frequently carried out to determine the results, various attempts have been made to automate the process of tests and measurements. In such tests and measurements, numerous types of samples each having little volume must be treated, and therefore, a step of dispensing, which involves a collection of a predetermined volume of the sample by suction, is inevitable. For example, in an immunoassay utilizing an antigen-antibody reaction, a specimen such as serum, plasma, urine and other body fluids collected from a living body is repeatedly dispensed into a number of reaction vessels, and the thus dispensed specimens are mixed or diluted with a reagent before the test results may be obtained. As described above, in the above-mentioned fields, numerous types of specimens must be repeatedly dispensed and diluted with reagents, and therefore, a number of attempts have been made to automate various steps of the dispensing.
In order to automate the dispensing step, an accurate detection of the liquid sample surface in the sample container is critical, since, in the automatic dispensing, the nozzle tip has to be inserted into the sample liquid to a predetermined depth to collect a predetermined volume of the sample into the nozzle tip. An inaccurate detection of the liquid surface in the container may result in a decreased dispensing precision. For example, when the liquid surface detected is higher than the actual surface, the depth of the nozzle tip inserted into the liquid would be decreased and air may be sucked into the nozzle tip to result in a significant decrease of the dispensing precision. On the contrary, when the liquid surface detected is lower than the actual surface and the nozzle tip is inserted into the sample solution further than the predetermined depth, an increased volume of the sample attached to the outer surface of the nozzle tip would be introduced into the reaction container, and also, the pressure exerted to the gas within the nozzle tip by the sample solution would be increased to change the gas volume within the nozzle tip to affect the volume of the sample collected. Consequently, the dispensing precision would be decreased
Accordingly, various methods for detecting the liquid surface as well as the system used therefor have been proposed together with sampling and dispensing methods and the systems used therefor.
Japanese Patent Application Kokai No. 56-164958 discloses an automatic dispenser wherein a negative pressure is applied to a cylinder which is in communication with a nozzle tip, and the surface of the sample liquid is detected by using a pressure difference induced between atmosphere and the interior of the nozzle tip upon contact of the nozzle tip with the surface of the liquid sample, and thereafter, a predetermined volume of the sample is collected by using the negative pressure of the cylinder.
Japanese Patent Application Kokai No. 62-64912 discloses a dispenser wherein a collection of the liquid sample by suction is started in response to a change of the pressure within the nozzle before and after the contact of the lower end of the nozzle tip with the liquid sample surface, and the volume of the sample dispensed is determined on the bases of the pressure within the nozzle after collecting the sample for a predetermined time.
Japanese Patent Application Kokai No. 63-109330 discloses a liquid surface-detector wherein air is discharged and sucked through a nozzle by a pump, and liquid surface is detected by a change of the pressure within the nozzle upon contact of the nozzle with the liquid surface. The discharge and the suction of the liquid surface-detecting air is carried out with a suction pump used for collecting the sample.
Japanese Patent Application Kokai No. 63-109373 discloses a sampling system wherein air is discharged and sucked through a sampling nozzle by a compressor, and liquid surface is detected on the bases of a change of the pressure within the sampling nozzle upon access or contact of the sampling nozzle with the liquid surface, and thereafter, the sample solution is collected through the sampling nozzle with suction by using a plunger pump.
In the above-mentioned conventional liquid surface-detectors and the liquid dispensers, the suction of the sample liquid is carried out after the liquid surface detection by reducing the pressure of the air used for the liquid surface detection in the nozzle, cylinder, gas conduit, pump and the like to exert a negative pressure to the sample liquid to thereby suck the sample liquid into the nozzle. Since the air is a compressible fluid, pressure control during the suction is quite difficult. More illustratively, the state of the air at the completion of the sample collection may vary in accordance with the volume and the pressure of the air, and the volume of the sample liquid collected rendering an accurate control of the volume of the sample collected difficult. In particular, a consistent, repeated suction of a small volume of sample with little variation from sample to sample as well as from system to system is quite difficult to achieve with the conventional dispenser system.
Even if an accurate volume of the sample liquid were collected into the nozzle tip, the thus collected sample must be discharged into a reaction vessel such as a test tube for the subsequent reaction or measurement of, for example, concentration, and upon such a discharge, a considerable amount of the sample liquid will remain attached to the nozzle tip to make it difficult to discharge all of the sample liquid in the nozzle tip. Consequently, the volume of the sample discharged would be inaccurate. For dispensing an accurate volume of the sample, the volume of the sample collected by suction into the nozzle tip must be determined by taking into account the volume of the sample which will remain attached to the nozzle tip upon discharge of the sample from the nozzle tip, and this would require a troublesome step of preliminarily wetting the nozzle tip with the sample to be dispensed.
Furthermore, the dispenser is primarily intended for dispensing the sample. Therefore, when a dilution of the sample, in particular, an accurate dilution of the sample is required, the once dispensed sample has to be diluted with a diluting reagent by dispensing the reagent.
A system which is capable of carrying out both the dispensing and the diluting steps is commercially available. This commercially available system, however, requires a manual liquid surface-detection, which may lead to a variation in the results from sample to sample. Also, the system is not fully automated since the liquid surface detection is manually conducted.